drm: make is_lspcon_adaptor static

Fixes sparse warning:

drivers/gpu/drm/drm_dp_dual_mode_helper.c:151:6: warning: symbol
'is_lspcon_adaptor' was not declared. Should it be static?

Fixes: 056996b9 ("drm: Helper for lspcon in drm_dp_dual_mode")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: <dri-devel@lists.freedesktop.org>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476789711-19697-1-git-send-email-jani.nikula@intel.com